Let them know your coming with Avenir's Dual Action Bell. It features the classic "ring-ring" sound and the steel top has a nice resonance. It's a whole lot better than yelling "on your left" 100 times.

Don't let the small size fool you. Mirrycle's Incredibell Triple is little, light and LOUD! Plus, its diminutive dimensions let you mount it just about anywhere on your handlebars. A flick of your finger and the Ding! Ding! rings clear for all to hear. The Triple fits the 3 most common handlebar sizes: 22.2mm, 25.4mm and 31.8mm.